The mercury vapor density or mercury vapor pressure in fluorescent lamps is an important parameter in determining lumen output and lamp efficacy. Low-pressure mercury vapor discharge lamps have a maximum efficiency of converting the electrical energy supplied into ultraviolet radiation at a predetermined vapor pressure. It is known that the envelope cold spot temperature for most efficient lamp operation is approximately 40.degree. C. This temperature causes a mercury vapor pressure of approximately 4 to 6.times.10.sup.-3 Torr to occur inside the lamp.
The mercury vapor pressure is typically very highly dependent on the temperature of operation of the lamp. Often, due to high lamp loading or high ambient temperature, the envelope temperature and mercury vapor pressure rise above the optimum value. As the temperature of lamp operation increases, the vapor pressure and density of mercury in the discharge lamp tend to increase above the desired levels required for optimum light output and efficiency of operation.
It is well known to control the mercury vapor pressure by use of an metal or metal alloy which forms an amalgam with mercury. As the mercury vapor pressure increases to an undesirable level, the amalgam begins to melt and form a solution with mercury vapor so as to decrease the vapor pressure of the mercury and return the lamp to a more optimal operation. Amalgams of indium and bismuth are known to possess these desirable properties.
It is also known that the location of the amalgam in the lamp is an important fact in providing the desired improvement. In practice, the lamp temperature can vary significantly relative to the desired stabilization of the lamp. Thus, the location of the alloy will effect its temperature and, in turn, the mercury vapor pressure. J. Bloem et al., "Amalgams for Fluorescent Lamps", Philips Tech. Rev., 38, 83-88 (1978) suggests that the location of amalgams can be important for lamp starting if the heat from the filament can be coupled to the amalgam. U.S. Pat. Nos. 3,869,772 to Latassa et al, 3,898,720 to Morehead, and 4,157,485 to Wesselink et al describe the glass stem as a desirable location for the amalgam which may be alloys of indium, bismuth, and tin. British Patent Specification No. 1,097,090 to Sylvania discloses positioning amalgam at certain locations in fluorescent lamps.
U.S. Pat. Nos. 4,977,349 to Asakura et al, and 4,288,715 to van Overveld et al, as well as U.K. application no. 2,157,883 describe compact type low-pressure mercury vapor discharge lamps having a folded path between the electrodes which employ an amalgam in the electrode stem area. In the '349 patent, a restriction in a receptacle aids in keeping the amalgam confined to the receptacle. However, during base up lamp processing amalgam has a tendency to fall into the lamp. When the ballast is included in the lamp base, the temperatures in the stem area may be undesirably high for the amalgam. U.S. Pat. No. 4,393,325 to van der Kooi describes a compact lamp with a build in ballast included in the lamp base and the amalgam positioned away from the stem area. The amalgam is in a separate metal container having a slit opening to the arc discharge area.
Although the above-described amalgam containing low-pressure mercury vapor discharge lamps have been employed with varying degrees of success, it has been found that certain disadvantages do exist. It is difficult to contain the amalgam during lamp processing and lamp operation when the lamp base is in an upward position. It is also difficult to incorporate the amalgam into the lamp at a location which is responsive to temperature at which the desirable absorption properties of the amalgam are advantageously utilized.